An optical disc device typically includes tracking servo control. The optical disc device emits a beam of light to a particular position of a track on the disc when the optical disc device records, replays or erases the data of the optical disc. The optical disc device obtains the tracking error by detecting the reflected beam from the spot on the optical disc. The tracking servo control corrects the tracking error to zero, hence the light spot falls on the accurate position of the track of the optical disc. There is a detailed description about the technology correcting tracking error in the U.S. Pat. No. 5,828,634.
FIG. 1A and FIG. 1B are schematic diagrams for explaining how to obtain a traditional track crossing signal. As shown in FIG. 1A, the pit track 112 and the land track 114 are disposed alternatively on the surface of the optical disc 110. Generally, the data are recorded in the pit track 112. The radial direction of the optical disc 110 is the track crossing direction which is presented as arrow “X”. The tangential direction of the optical disc 110 is the rotating direction which is presented as arrow “Y”.
The traditional three-beam method for obtaining the track crossing signal is to use a laser to impinge on the surface of the optical disc 110 and generate three spots on the surface of the optical disc 110. The main spot 117, the subordinate spot 116 and the other subordinate spot 118 are substantially located in a line. The difference in brightness of the reflected light corresponding to the subordinate spots 116 and 118 approaches to zero when the main spot 117 falls on a pit track 112.
As shown in FIG. 1B, when the main spot 117 falls on the track crossing position, the half part of the main spot 117 falls on the pit track 112 and the other half part falls on the land track 114. There is a maximum value of the absolute value of the difference in brightness of the reflected lights corresponding to the subordinate spots 116 and 118. By utilizing the difference in brightness, the track crossing signal can be obtained.
FIG. 1C is a schematic diagram of a traditional track crossing signal observed on the oscilloscope. When the optical disc 110 is rotating and the optical pickup is fixed, the reflected lights corresponding to the subordinate spots 116 and 118 are read, transformed, and operated to generate a track crossing signal 120. The track crossing signal 120 shown on the oscilloscope includes a continuous sine wave and cosine wave and the wave number and the amplitude relate to the parameters which are set in the oscilloscope. A wave envelope of wave peak 122 can be obtained by connecting several wave peaks of the track crossing signal 120. Also, a wave envelope of wave troughs 124 can be obtained by connecting several wave troughs of the track crossing signal 120.
Point “A” is on the wave envelope 122 of wave peaks, point “B” corresponds to point “A” and is on the wave envelope 124 of wave troughs. The difference of amplitude of these two points is a maximum “Am1”, and the maximum “Am1” corresponds to the maximum absolute value of the difference in brightness of reflected lights corresponding to the subordinate spots 116 and 118 as shown in FIG. 1B.
However, the defect of the optical disc 110, such as an eccentric optical disc, makes the resulting track crossing signal different from the track crossing signal 120 shown in FIG. 1C. The mechanism and quality of the components of the optical disc device also cause the track crossing signal to be different from the track crossing signal 120 shown in FIG. 1C. Thus, it is desired to improve the disc-reading capability of the optical disc device.